An image forming apparatus using electrophotography may include a plurality of image carriers (e.g., photoconductor) and a transfer member (e.g., transfer belt) facing the image carriers, in which the transfer member may travel in an endless manner in one direction.
In such image forming apparatus, toner images having different color may be formed on each of the image carriers.
Such toner images may be superimposingly transferred onto the transfer member, and further transferred onto a recording medium (e.g., transfer sheet), by which a full-color toner image may be formed on the recording medium.
In such configuration, toner images may not be correctly superimposed on the recording sheet in a sub-scanning direction of image forming direction by several factors in some cases.
Such factors may include a deviation of light-path in an optical unit from a normal path due to a temperature change, relative positional changes of the image carriers due to an external force, for example.
If toner images may not be superimposed correctly on a recording medium when forming a fine/precise image by superimposing a plurality of color toner images, image dots having different color may not be superimposed correctly on the recording medium, by which a resultant image may have a blurred portion, which may not be acceptable as fine/precise image.
Furthermore, if such incorrect superimposing may occur when forming a character image on a non-white sheet, a white area may occur around the character image.
Furthermore, if such incorrect superimposing may occur when forming an image having a plurality of colored areas on a sheet, a white area may occur at a border of different colored areas or an unintended color image area may occur at a border of different colored areas.
Furthermore, if such incorrect superimposing may occur when forming an image having a plurality of colored areas on a sheet, unintended stripe images may occur on a sheet, and cause uneven concentration on an image, which is printed on the sheet.
Such phenomenon may unfavorably degrade an image quality to be formed on the recording medium.
Such drawback that toner images may not be correctly superimposed on the recording sheet in a sub-scanning direction of image forming direction may be reduced or suppressed by adjusting a writing timing of an optical unit of an image forming apparatus.
Hereinafter such drawbacks may be referred to “superimposing-deviation of images” or “superimposing-deviation,” as required, for the simplicity of expression.
An adjustment of writing timing of the optical unit may be conducted as below.
At first, a toner image may be formed on each of the image carriers (e.g., photoconductor) at a given timing, and then transferred onto to a surface of a transfer member such as transfer belt as detection images.
Such detection images may be used to detect an image-to-image positional deviation between toner images, to be formed on the transfer member.
A photosensor may sense the detection images and transmits a signal, corresponding to each of the detection image, to a controller of the image forming apparatus. The controller may judge a detection timing of the detection image based on the signal.
The controller may compute a relative image-to-image positional deviation value between each of the toner images based on the signal.
Based on computation by the controller, the controller may set a starting timing for writing a latent image on each of the image carriers (e.g., photoconductor) independently, by which a superimposing-deviation of images may be suppressed.
The above-mentioned image forming apparatus may employ a direct transfer method, which transfers toner images from image carriers to a recording medium, which may be transported by a transport belt.
The above-mentioned image forming apparatus may also employ an intermediate transfer method, which transfers toner images from image carriers to a transfer belt, and further to a recording medium.
In both of such configurations, adjusting a writing timing of an optical unit may reduce a superimposing-deviation of images.
Toner images may not be correctly superimposed on the recording medium by the above-mentioned factors such as a deviation of light-path in an optical unit due to a temperature change, and relative positional changes of the image carriers due to an external force, for example. In addition to such factors, other factors may cause an incorrect superimposing of toner images.
Other factors may include an eccentricity of image carrier, an eccentricity of drive-force transmitting member (e.g., gear) that rotates with image carrier, and an eccentricity of a coupling member that is connected to image carrier, for example.
Specifically, if the image carrier or drive-force transmitting member may have an eccentricity, the image carrier may have two areas (e.g., first and second areas) on the surface of the image carrier with respect to a diameter direction of the image carrier.
For example, the first area of the image carrier may rotate with a relatively faster speed due to the eccentricity, and the second area of the image carrier may rotate with a relatively slower speed due to the eccentricity, wherein such first and second areas may be distanced each other with 180-degree with respect to a diameter direction of the image carrier, for example.
In such a case, first image dots formed on the first area of the image carrier may be transferred to a transfer member at a timing earlier than an optimal timing, and second image dots formed on the second area of the image carrier may be transferred to the transfer member at a timing later than an optimal timing.
If such phenomenon may occur, first image dots formed on one image carrier may be superimposed on second image dots formed on another image carrier. Similarly, second image dots formed on one image carrier may be superimposed with first image dots formed on another image carrier.
Such phenomenon may cause incorrect superimposing of toner images having different colors in a sub-scanning direction.
The above-mentioned adjusting control work may adjust an optical writing position for each photoconductor in a sub-scanning direction in one image, but may not adjust a speed variation in one photoconductor, by which “superimposing-deviation of images” may not be suppressed or reduced.
In another image forming apparatus, a controller may conduct a speed-variation detection control and a phase adjustment control for toner images to reduce an incorrect superimposing of toner images.
The speed-variation detection control may be conducted by detecting a deviation of surface speed of an image carrier (e.g. photoconductor), which may occur when conducting an image forming operation.
The phase adjustment control may be conducted by adjusting a phase of each image carrier based on the speed-variation detection control.
In case of speed-variation detection control, a plurality of toner images may be formed with a given pitch each other on a surface of one image carrier in a surface moving direction of one image carrier.
Such plurality of toner images may be then transferred to a transfer member (e.g., transfer belt) as speed-variation detection image, and a photosensor may detect each of the toner images included in the speed-variation detection image.
Based on a detection result by the photosensor, a pitch of toner images included in the speed-variation detection image per one revolution of one the image carrier (e.g., photoconductor) may be computed.
Based on the computed pitch, a speed variation per one revolution of one image carrier may be determined.
Furthermore, another photosensor may detect a marking placed on a gear, which may drive the image carrier, to detect a timing that the image carrier comes to a given rotational angle.
With such process, the controller of the image forming apparatus may compute a difference between a first timing when the image carrier comes to the given rotational angle and a second timing when the surface speed of image carrier becomes a maximum or minimum speed.
Such speed-variation detection control process may be conducted for each of the image carriers.
After conducting such speed-variation detection control, a phase adjustment control may be conducted to adjust a phase of image carriers.
Specifically, a photosensor may detect a marking placed on a given position of a gear, which rotates the image carrier.
A plurality of photosensors may be used to detect a marking placed on a given position of gears, which drives respective image carriers.
With such process, a timing when each of the image carriers is disposed at a given rotational angle may be detected.
Based on a comparison of a timing for such given rotational angle and a timing detected by speed-variation detection control process for each of image carriers, a plurality of drive motors, which respectively drives each of the image carriers, is driven by changing a driving time period temporarily to adjust a phase of image carriers.
With such phase adjustment of image carriers, image dots that may come to a transfer position at a timing earlier than an optimal timing, or image dots that may come to a transfer position at a timing later than an optimal timing, may come to a transfer position at an optimal timing. With such controlling, a superimposing-deviation of images may be reduced.
Furthermore, if a pitch between adjacent image carriers may be set to a value, which is equal to a length obtained by multiplying a circumference length of image carrier with an integral number (e.g., one, two, three), each of the image carriers may rotate for an integral number (e.g., one, two, three) during a time when one toner image is transferred from one image carrier to a sheet at one transfer position and then moved to a next transfer position on a next image carrier.
Accordingly, under such configuration, by adjusting a phase difference of image carriers to substantially “zero” level, image dots may be better transferred to a transfer member at each transfer position.
On one hand, if a pitch between adjacent image carriers may not be set to a value, which is equal to a length obtained by multiplying a circumference length of image carrier with an integral number (e.g., one, two, three), each of the image carriers may not rotate for an integral number (e.g., one, two, three) during a time when one toner image is transferred from one image carrier to a sheet at one transfer position and is moved to a next transfer position on a next image carrier. In such a case, a different phase may be set for each of the image carriers respectively, by which image dots may be transferred to a transfer member from each of the image carriers at each transfer position, defined by the transfer member and the each of the image carriers.
In addition to the above-explained method using a phase adjustment control, which may adjust a phase relationship of each of photoconductors having a speed variation in each of photoconductors, for suppressing superimposing-deviation of images due to an eccentricity of each photoconductor, another method (namely, adjusting a driving speed of a drive motor) may be used for suppressing superimposing-deviation of images due to an eccentricity of each photoconductor.
In such another method, the drive motor may be driven in a pattern, which may be an opposite phase relationship with a speed variation pattern, in which a speed variation of photoconductors may be suppressed by adjusting a driving speed of drive motor.